A Universal Serial Bus (USB) standard has been developed which allows up to 127 peripheral devices such as printers, scanners, keyboards, modems, cameras and storage devices to be attached to a host, usually a personal computer (PC), through a 4-wire bus. These devices can be connected to the PC either directly or via hubs which provide additional connections. USB has the advantage that connection of different types of devices becomes standardised. Furthermore, a device can be connected while the PC is switched on and while other devices are in use. USB supports user data rates of up to 8 Mbit/s for each device which is suitable for low to medium data rate applications. An overview of USB may be found in “USB Hardware & Software” by J. Garney et al. (Anna Books, 1998) [ISBN 0-929392-3-X]and a current version of the USB specification is available at www.usb.org or from USB-IF Administration, 5440 SW Westgate Drive, Suite 217, Portland, Oreg. 97221 USA.
A device is connected to a USB port provided by the PC or a hub. Once physically connected to the device, the PC controls attachment and configuration of the device. To achieve this, the PC is installed with a USB driver which is usually provided by the PC's operating system. The PC is also installed with a device driver to control the device once it is attached and configured so that application software on the PC can use the device. The device driver is often provided by the operating system although for unusual devices, a user may need to install a specific device driver using installation disks.
Devices fall into two main categories, namely hubs and functions. Hubs provide additional attachment points to the USB. Functions extend the capabilities of the computer. Most devices which are not a hub, such as a mouse, implement a single function. Some devices, such as a monitor having in-built speakers, implement multiple functions and have an embedded hub. Such a device is known as a compound device and appears to the PC as a hub with a collection of individual, non-removable functions.
The USB standard was originally designed to accommodate simple peripheral devices which fulfil a single function. When a single function device, such as a mouse, is plugged into a PC for the first time, the USB driver detects, identifies and configures the device and the operating system automatically assigns a device driver, which in the case of a mouse is a mouse driver. Alternatively, a user may install and/or assign a specific device driver. When a compound device is plugged in for the first time the same process of detection, identification and configuration is carried out for each respective function so that all the functions of the compound device are available to the PC.
However, a problem arises if a device implements multiple disconnectable functions. Such a device appears to the PC as a single function device. When the device is plugged into the PC for the first time, the operating system assigns a device driver according to this function. Once assigned, the device is tied to the device driver so that whenever the device is disconnected from the PC and then reconnected, the same device driver is assigned to the device. This abrogates the multifunctionality of the device.
The present invention seeks to help overcome this problem.